There has been used an electric power steering system configured to drive an electric motor according to steering torque exerted on a handle (a steering wheel) by a driver, thereby providing auxiliary steering force to a steering mechanism of a vehicle. A brush motor has been widely used as an electric motor of the electric power steering system. Recently, brushless motors are also used from the viewpoint of improvements in reliability and durability and the reduction of inertia, etc.
In order to control torque generated in a motor, a motor controller usually detects an electric current flowing through the motor and performs PI control operation (a proportional-integral control) in accordance with a difference between an electric current to be supplied to the motor and the detected electric current. The motor is, for example, a three-phase brushless motor, and is driven by three types of PWM signals having a duty ratio based on levels of three-phase voltages. The motor controller that drives the three-phase brushless motor includes two or three current sensors for detecting electric currents having two phases or more.
In relation to the present invention, JP-A-8-317698 discloses a configuration for determining a d-axis command voltage and a q-axis command voltage by use of a motor circuit equation and correcting the d-axis command voltage and the q-axis command voltage during regenerative operation in order to improve the torque characteristics. JP-A-2004-64839 also discloses a configuration for estimating a state of motor load from an electric current flowing through the motor, thereby correcting the duty ratio for the PWM signals. Moreover, JP-A-2007-166711 also discloses a configuration for correcting the duty ratio according to a supply voltage.